Regarding the fuel injection control in engines, a common-rail type fuel injection system has been known which provides a high injection pressure and performs optimum control on injection conditions, such as fuel injection timing and the amount of fuel injected, according to the operating condition of the engine. The common rail type fuel injection system is a system that stores in the common rail a fuel pressurized to a predetermined pressure by a fuel pump and then injects the stored high-pressure fuel into corresponding combustion chambers from injectors under the control of a controller. Fuel flow paths extending from the common rail through branch pipes to nozzle holes of individual injectors are acted upon at all times by a fuel pressure corresponding to the injection pressure. The controller controls the individual injectors so that the pressurized fuel is injected from each injector under an optimum injection condition according to the operating state of the engine.
An outline of the common-rail type fuel injection system is shown in FIG. 12. In the common-rail type fuel injection system, the fuel is supplied from the common rail 2 through branch pipes 3 forming a part of the fuel flow paths to injectors 1 that inject fuel into corresponding combustion chambers. The fuel, which was pumped by a feed pump 6 from a fuel tank 4 through a filter 5, is delivered through a fuel pipe 7 to a fuel pump 8 which, for example, is a variable-displacement high-pressure pump of plunger type. The fuel pump 8 is driven by the engine to raise the pressure of the fuel to a required predetermined pressure and supply the fuel to the common rail 2 through a fuel pipe 9. The fuel pump 8 maintains the fuel pressure in the common rail 2 at a predetermined pressure. The fuel released from the fuel pump 8 is returned to the fuel tank 4 through a return pipe 10. Of the fuel supplied from the branch pipes 3 to the injectors 1, the fuel that was not used for injection into the combustion chambers is returned to the fuel tank 4 through a return pipe 11.
The controller 12 as an electronic control unit is supplied with signals from various sensors for detecting the engine operating condition, which include an engine revolution speed sensor 40 to detect an engine revolution speed Ne, an engine cylinder determination sensor 41, a top dead center (TDC) detection sensor 42, an accelerator pedal depression amount sensor 43 to detect the amount of accelerator pedal depression Acc, a cooling water temperature sensor 44 to detect the temperature of cooling water Tw, an atmospheric temperature sensor 45 to detect the temperature of atmosphere Ta, an atmospheric pressure sensor 46 to detect the pressure of atmosphere Pa, and an intake pipe inner pressure sensor 47 to detect the inner pressure of the intake pipe Pb. The controller 12, based on these signals, controls the fuel injection conditions of the injectors 1, i.e., the fuel injection timing and the amount of fuel to be injected, so that the engine output will become optimum for the engine operating condition. The common rail 2 is provided with a pressure sensor 13 which detects a fuel pressure Pc in the common rail 2 and sends the detection signal to the controller 12. The fuel pressure in the common rail falls when the fuel in the common rail 2 is consumed by the injectors 1 injecting the fuel. The controller 12 controls the amount of fuel delivery from the fuel pump 8 so that the fuel pressure in the common rail 2 remains constant.
FIG. 13 shows a cross section of the injector 1. The injector 1 is mounted hermetically, through a seal member, in a hole portion provided in a base such as cylinder head. The structure of the cylinder head is not shown. The side portion of an upper part of the injector 1 is connected with a branch pipe 3 through a fuel inlet joint 20. The injector 1 has fuel passages 21, 22 formed therein, and the branch pipe 3 and the fuel passages 21, 22 together form fuel flow paths. The fuel supplied from the fuel flow paths flows past a fuel sump 23 and a passage around a needle valve 24 and is injected into the combustion chamber from nozzle holes 25 that are opened when the needle valve 24 is lifted.
The injector 1 is provided with a balance chamber type needle valve lift mechanism that controls the lift of the needle valve 24. That is, at the uppermost part of the injector 1 is provided a solenoid valve 26 whose solenoid 28 is supplied with a control current as a control signal from the controller 12 through a signal line 27. When the solenoid 28 is energized, an armature 29 is lifted to open an on-off valve 32 provided at the end of a fuel passage 31, through which the fuel pressure supplied to a balance chamber 30 is released. The injector 1 has a hollow space 33 formed therein, in which a control piston 34 is installed vertically movable. Because a push-down force acting on the control piston 34 which is a combined force of the reduced inner pressure in the balance chamber 30 and the spring force of a return spring 35 is exceeded by a push-up force acting on the control piston 34 which is produced by the fuel pressure acting on a tapered surface 36 facing the fuel sump 23, the control piston 34 moves up. As a result, the needle valve 24 is lifted injecting fuel from the nozzle holes 25. The amount of fuel injected is determined by the fuel pressure in the fuel flow paths and the lift (the amount and duration of the lift) of the needle valve 24. The lift of the needle valve 24 is determined by an injection pulse as a control current sent to the solenoid 28 which controls the on-off operation of the on-off valve 32.
FIG. 14 shows the relation between the amount Q of fuel injected from the injector 1 and the width W of a command pulse supplied from the controller 12 to the solenoid 28, with the fuel pressure Pc (fuel pressure in the common rail 2) as a parameter. If the fuel pressure Pc is taken to be constant, the fuel injection amount Q increases with the command pulse width W. For the same command pulse width W, the fuel injection amount Q increases as the fuel pressure Pc increases. The fuel injection starts or stops with a certain time delay after the command pulse has risen or fallen. Thus, controlling the timing at which the command pulse is turned on or off enables the injection timing to be controlled.
The amount of fuel to be injected in each combustion cycle is calculated from a basic injection amount characteristic map shown in FIG. 15. FIG. 15 shows how a basic injection amount Qtb changes according to the engine revolution speed Ne with the abscissa representing the engine revolution speed Ne and the ordinate representing the basic injection amount Qtb and with the accelerator pedal depression amount Acc taken as a parameter changing to various values. As shown in FIG. 15, the characteristic map is so set that when, with the accelerator pedal depression amount Acc kept constant, the engine revolution speed Ne increases, the basic injection amount Qtb decreases. Hence, when the engine revolution speed Ne increases for some reason, the feedback control reduces the amount of fuel to be injected according to the basic injection amount Qtb, causing the engine revolution speed Ne to be reduced. As a result, the engine revolution speed will stabilize at a fuel injection amount that balances with the internal resistance of the engine.
In the fuel injection control device for engines, the following proposals have been made as measures to control the fuel injection timing and amount with high precision. That is, in a system where the fuel injection is controlled based on a reference timing and an injection period from the reference timing, it is proposed that a dummy injection device be provided separate from the engine cylinders and that the actual injection amount from the dummy injection device be detected and used to determine the amount of fuel to be injected in order to prevent the fuel injection amount from being changed greatly by small variations of the engine revolution speed (see Japanese Patent Laid-Open No. 182460/1987).
A high-pressure fuel delivery under pressure by the fuel supply pump, a pressure reduction at times of injection, and a water hammer action from valve closure at the end of injection cause pulsations in the common rail pressure. It is known from experience that even during the pulsations the common rail pressure at the trailing edge of the command pulse for the fuel injection valve is almost equal to the actual injection pressure. Taking advantage of this fact, it has been proposed that the common rail pressure at the trailing edge of the command pulse be sampled to determine the amount of fuel to be injected (see Japanese Patent Laid-Open No. 125985/1993).
Further, in a common-rail type fuel injection control device which, based on the detected value of the operating condition parameter such as engine revolution speed and accelerator pedal opening and the detected value of the injection pressure in a cylinder that has finished injection in a previous cycle, calculates an injection pressure command value for the cylinder to be used in the next injection cycle and performs fuel injection for an injection period corresponding to this injection pressure command value; it is proposed that when the engine is in a transient state, an instantaneous change in the fuel injection pressure corresponding to a crank angle be calculated to correct the injection pressure for the cylinder used to determine the fuel injection period that will be used in the next injection cycle, thereby improving the precision of the fuel injection control during the transient state (see Japanese Patent Laid-Open No. 93915/1994).
These common-rail type fuel injection control devices described in the above official gazettes attempt to improve the precision of the fuel injection from a variety of standpoints but do not consider variations of fuel injection among cylinders. That is, in the common-rail type fuel injection systems, the rate of fuel injected from the injectors depend on the common rail pressure, nozzle hole diameter, the speed at which the needle valve is opened, and the throttle of the fuel flow paths. The common rail pressure is common to all injectors while other factors including the nozzle hole diameter, the needle valve's opening speed and the throttle of the fuel flow paths differ from one injector to another. Thus, even when the operating states of the solenoid valves used for the control of the lift of the needle valves in the injectors are made equal, inevitable variations in the fuel injection rate characteristics such as the fuel injection start timing, the fuel injection rate and the maximum fuel injection pressure render uniform control among the cylinders difficult.
As for the variations in fuel injection among the injectors, detailed descriptions will be given with reference to FIG. 16 that illustrates changes over time of the fuel injection rate. The graph of FIG. 16 shows the fuel injection rate when the energization times of the solenoid valves of the injectors in a 6-cylinder engine are made equal. The figure shows the fuel injection rates of two injectors between which a largest injection rate difference exists, and also an average fuel injection rate of the six injectors. There are the following three factors that can cause variations in the fuel injection rate among the injectors. As to the fuel injection start timing, there is a variation of about 1.5 degrees in crank angle CA as shown at A in the figure; regarding the amount of fuel injected during the initial injection period (ignition delay period) tf, there is a relative variation of about 30% as shown at B; and as to the maximum injection rate, there is a relative variation of about 15% as shown at C.
When a single engine has such variations in the fuel injection characteristics among the injectors installed in the corresponding cylinders, it is impossible to obtain optimum injection timing and fuel injection amount for each injector, which in turn degrades the cleanliness of the exhaust gas and causes combustion imbalance among the cylinders, resulting in noise and vibrations.
The variations in the fuel injection characteristics are considered to be caused by variations in the machining and assembly precision including dimensional and coarseness precision during the course of manufacture of the constitutional parts, such as injector nozzle hole diameters, needle valve opening speed and fuel flow path throttle. These variations are unique to each injector, and to reduce them uniformly among the injectors requires further improvement of the machining and assembly precision of the injector components. Improving these precisions, however, gives rise to another problem of increased manufacturing cost because it requires modifying production facilities.
If, when injectors have injection characteristics variations among them, the injection characteristics can be corrected in a way that reduces injection characteristics variations among the injectors, it should be possible to perform control so that the injection characteristics are uniform among all of the injectors, without having to take a drastic measure of changing the production facilities--a factor that contributes to increased cost--to make further improvements in the machining and assembly precision of the injector components.
An object of this invention is to solve the above-described problems and to provide a fuel injection control method and device which, by taking advantage of the fact that the fuel injection of each injector is electronically controlled, eliminates variations in the injection characteristics among the injectors based on data obtained by time-differentiating the common rail pressure and thereby controls the injection timing and the amount of fuel to be injected so that the injection characteristics of all of the injectors used will be uniform.
If, of variations in the fuel injection characteristics, the fuel injection start timing variations in terms of crank angle CA can be limited to within 0.2 degrees, the fuel injection amount variations during the ignition delay period can be limited to within .+-.5%, and the maximum injection rate variations can be limited to within .+-.2%, the uniformity of combustion among the cylinders can be maintained. This prevents deterioration of cleanliness of exhaust gas and maintains the combustion balance among the cylinders, which in turn keeps noise and vibrations from deteriorating.